Pencil
Pencil is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics.
Synfig
Synfig Studio is a free and open-source 2D animation software, designed as powerful industrial-strength solution for creating film-quality animation using a vector and bitmap artwork. It eliminates the need to create animation frame-by frame, allowing you to produce 2D animation of a higher quality with fewer people and resources. Synfig Studio is available for Windows, Linux and MacOS X.